The present invention relates to the art of digital communications, particularly to radio navigation, and more precisely to satellite based radio navigation.
Global Navigation Satellite Systems (GNSS), such as the US GPS, the European GALILEO and the Russian GLONASS, are based on a mathematical concept known in the art as “three dimensional Trilateration”, where a point is determined by its distances from three other points. The point we wish to determine is the position of a GNSS receiver, typically located by the earth surface, in a car or onboard a ship or aircraft or carried by a person, while the other points are satellites orbiting around the earth. The distances between the satellites and the receiver are estimated by measuring the travelling time of signals transmitted from the satellites, at the speed of light, until arriving at the receiver. FIG. 1 depicts the GPS Trilateration Concept.
Theoretically, three distance measurements are required to resolve the three spatial coordinates (x, y, z) of the receiver. However, practically, four satellites are monitored by the receiver, in order to account also for the receiver's clock drift, compared to the accurate satellites and system clocks.
The four basic GPS equations, referred in the art as the “navigation equations” or “range equations” or “pseudo range equations”, are represented as following:PRi−C*ΔtSVi=SQRT[(x−xi)2+(y−yi)2+(z−zi)2]+C*ΔtR; i=1-4  (1)
Where:
PRi=“Pseudo Range” between SVi (Space Vehicle i) and the receiver
C=the speed of light in free space, ˜300,000 Km/sec
ΔtSVi=SVi clock deviation from “GPS Time” (the system reference time)
x, y, z=the receiver's position (unknown, to be calculated)
xi, yi, zi=SVi position
ΔtR=the receiver's clock drift from GPS Time.
As a person skilled in the art may appreciate, the part SQRT[(x−xi)2+(y−yi)2+(z−zi)2] expresses the geometrical distance between SVi and the receiver, in Cartesian coordinates. This expression is actually the Pythagorean theorem in three dimensions.
On the other side of equation (1), PRi also expresses the range between SVi and the receiver, however as hinted by its name, PRi is not the exact range but only an approximated range. PRi is the travelling time between SVi and the receiver, multiplied by the speed of light, yet the travelling time is estimated as the difference between detection time instant to transmission time instant of the signal, however those time instants are measured by different clocks: the receiver's clock and the satellite's clock (the transmission time instant is broadcast by the satellite). The difference between these clocks is treated in the navigation equations (1), by accounting for each clock deviation from a common and accurate clock, administered by the GPS system, named “GPS Time”. Thus, ΔtSVi which stands for the SVi clock deviation from the “GPS Time”, and ΔtR which stands for the receiver's clock deviation from the same “GPS Time”, complement equations (1).
The basic task of a typical GNSS receiver is to resolve the four equations (1), to determine the four unknowns: x, y, z and ΔtR. Prior to resolving equations (1), the receiver has to determine the “known parameters” in equations (1), i.e. PRi, ΔtSVi, xi, yi, zi. For this purpose, the receiver uses information comprised in the reference signals broadcast by the GPS satellites.
As a person skilled in the art probably appreciates, the reference signals broadcast by GPS satellites are basically comprised of one or more RF carriers, modulated by two types of data streams: Pseudo-Random-Noise (PRN) codes and the navigation message.
The PRN codes are pre-known series of data, cyclically transmitted by each satellite, for synchronization and ranging purposes. Different satellites are allocated with different codes.
PRNs obtain sharp auto-correlation (i.e. correlation with same code shifted in time) and flat cross-correlation (i.e. correlation between different codes) properties. Since the receiver knows in advance exactly which satellite transmits which code, it generates a replica of this code, and a correlation between this replica and the received signal means that a specific satellite signal is detected, at a specific time instant.
The navigation message is a series of bits, organized in frames and sub frames, conveying navigational data to the receiver for resolving the navigation equations (1). Among other, the navigation message comprises information indicating the location of the GPS satellites, and information indicating the time instant when the signal was transmitted. The latter is used to determine the pseudo-range (PRi), between a satellite (i) and the receiver. The navigation message comprises also the satellite clock correction information, required to determine ΔtSVi.
PRi is calculated as the travelling time between SVi and the receiver, multiplied by the speed of light. The transmitter and the receiver agree upon a reference point, in the navigation message stream, for which the transmitter reports the transmission time. In GPS, this point is the first bit of the subframe that follows the subframe where the transmission time is reported. When the receiver detects this bit, it records its own time, say tRi, then decodes the transmitted time reported by SVi in the previous subframe, say tTi, and determines PRi=C*(tRi−tTi). However, since the navigation message bits are transmitted at a relatively low rate, typically 50 bps, their rise time is relatively long so provide a poor resolution of the estimated receive time. In order to improve the resolution of the receive time measurement, the PRN code, at a rate of 1.023 MHz (C/A signal), is used. This code is broadcast by the satellite, synchronized with the navigation message bits, therefore, when the receiver's correlator detects a PRN code, it actually refines tRi, to a level of about 1% of the PRN bit period, i.e. to 10 ns, equivalent to 3 meters in pseudo range. PR determination accuracy is one of the significant factors that influence the position determination accuracy of a GPS receiver. Typically, at the beginning of 2011, the GPS C/A service provides position accuracy better than 10 meters.
Determining the precise satellite position (xi, yi, zi) at the transmission time, is not straightforward. A GPS satellite does not report its instantaneous position, but reports parameters of a mathematical model that describes its orbit, from which its position can be calculated, for any time instant. These parameters are known in the art as the “Keplerian Elements” of the orbit. FIG. 2 illustrates the satellite orbit and the Keplerian Elements.
According to Kepler's 1st law, GPS satellites obtain an elliptical orbit with the center of the earth at one of the ellipse foci (plural of focus). In order to determine this orbit, six Keplerian elements are typically used: 2 parameters that describe the orbit shape and size, 3 parameters that describe the orbit orientation in space, and 1 parameter to determine the momentary position of the satellite on its orbit at one specific time. These parameters are repeatedly broadcast by each GPS satellite, as part of the ephemeris in the navigation message, and been updated every couple of hours or so.
The 6 Keplerian elements describing a GPS satellite orbit are:                i. a=semi-major axis of the ellipse        ii. e=eccentricity of the ellipse        iii. i=inclination between the orbit plane and the earth equator        iv. Ω=Right Ascension of Ascending Node (RAAN)=the spatial Longitude of the ascending node of the orbit        v. ω=argument of perigee=angle from the ascending node to point of closest approach        vi. toe=epoch of perigee passage=time when satellite is at perigee        
Basically, a GPS receiver detects the Keplerian elements (and further corrections) broadcast by satellites that it tracks, and calculates the momentary position of the satellite (xi, yi, zi), relevant for the navigation equations.
A more comprehensive description of the reference signals broadcast by GPS satellites can be found in the GPS Interface Specification (IS) documents, and in the GPS Interface Control Documents (ICD), published by US authorities. See—http://www.gps.gov/technical/icwg/
Both, IS-GPS-200E and IS-GPS-800A, dated 8 Jun. 2010, are references to the present invention.
Satellite navigation systems such as GPS, GALILEO and GLONASS, are designed to operate in open spaces, where there is substantially a line of sight between the receiver and the satellites. This is due to the relatively high frequency of the carrier of the signal broadcast by the satellites, typically in L-band. Therefore, the satellite signals can be hardly detected indoors.
In order to navigate with a GNSS receiver indoors, the present art suggests deploying an infrastructure of local transmitters that emulate satellite signals. Such local transmitters are known in the art as “Pseudolites” (“pseudo-satellites”). Pseudolites are most often small transceivers used to create a local, ground-based GPS alternative. The range of each transceiver's signal is dependent on the power available to the unit. Being able to deploy one's own positioning system, independent of the GPS, can be useful in situations where the normal GPS signals are either blocked/jammed (military conflicts), or simply not available (exploration of other planets), or applied to precision approach landing systems for aircraft and highly accurate tracking of transponders. In particular, Pseudolites gain more and more attention in the context of indoor location.
In large buildings, particularly multi floor buildings, deploying an infrastructure of pseudolites is problematic. Since GPS signals can hardly cross floors and walls, every floor and almost every room would require a dedicated set of pseudolites, and since these sets should be synchronized which each others, a huge cabling network is required or alternatively dense wireless transmissions.
Another issue concerning pseudolites is that they cannot emulate any imaginary satellite position, if a standard GPS receiver is required to detect these signals, but should be based on the Keplerian model.
Thus, the present art studies several other alternatives for indoors navigation, particularly using communication infrastructure densely deployed in urban areas, such as cellular and WLANs.
Most of these indoor navigation methods are also based on the Trilateration concept employed by GPS, wherein base stations of cellular or Access Points of WLANs replace the GPS satellites.
Such base stations may broadcast their own position, and assuming time synchronized networks, a mobile device could measure the Time of Arrival (TOA) of the signal broadcast by a cellular base station, and accordingly determine the pseudorange (or even range, if round trip signaling is feasible) to that base station. Fourth generation (4G) cellular standards such as LTE are quite concerned about these features. Similar methods can be applied in wireless LAN networks, such as WIFI or WiMAX.
Yet, in order to determine a position via Trilateration/TOA with cellular/WLAN networks, at least three such base stations should be simultaneously detected. However, typically, cellular/WLAN networks are not deployed so redundantly since differently than GPS, access to one base/reference station is enough for a mobile to communicate.
Furthermore, the Trilateration accuracy is very sensible to the geometry of the reference stations. As well known in the art, a poor geometry (i.e. low volume formed by the positions of the satellites and user) causes poor (high) DOP (dilution of position). In this context, cellular/WLAN Trilateration is expected to suffer worse DOP than GPS, due to the typically common level deployment of base stations. This poor nature of cellular/WLAN infrastructure particularly downgrades the height (or elevation or altitude) accuracy, in a way that such methods could hardly distinguish between near floors in a high building.
Therefore, it is an object of the present invention to provide a method for height determination by GNSS/GPS receivers operating indoors.
It is another object of the present invention to enable GPS receivers to determine self height indoors, at least distinguishing between floors.
It is then an object of the present invention to enable GPS receivers to determine self height indoors, deploying a modest infrastructure.
It is yet another object of the present invention to provide a method to determine the height or depth in vertically shaped spaces, such as mines, wheels and elevator shafts.